Integrated circuits continue to be designed with increasing numbers of signal lines for higher performance and lower cost per square inch. In addition, larger numbers of power supply and ground lines are needed to ensure that the circuits function at higher speeds. Traditionally, the electrical and mechanical connections between the integrated circuit (IC) package and the printed circuit board (PCB) were made by soldering and have been peripheral to the package. Usually these connections were along two parallel sides or on all four sides of a square or rectangular IC package.
As the number of lines into and out of IC packages has increased, more leads have been placed in less space. The dual inline package of the 1970s and 1980s had a typical lead spacing (also known as pitch) of 2.54 mm (0.100 inch), and the leads were soldered into copper plated holes in the PCB. Surface mount technology (SMT), which became more common for PCB assembly by the electronics industry during the 1980s and 1990s, eliminated the plated through holes for circuit leads and began the trend toward more closely spaced leads. Some of the first SMTs had 1.27 mm (0.050 inch) pitch components and by the 2000s, pitches had moved to 0.5 mm (0.0197 inch) for quad flat packs (QFPs). Assembly process tolerances and PCB fabrication limitations have limited widespread commercial manufacturing of components with pitches much smaller than 0.5 mm (0.0197 inch).
By changing from peripheral leads to solder balls (or columns) placed in an array under the IC package, many more leads with a larger pitch than those of a comparable QFP could be put on an IC package of any particular size. Electrical circuit paths are also shorter on these array packages than on comparable QFPs, resulting in improved electrical performance of high-speed circuits.
The most common forms of these array packages are called plastic or ceramic ball grid arrays (PBGA, CBGA), ceramic column grid arrays (CCGA), and chip scale packages (CSP). The same technology is also used for directly attaching silicon IC's to the PCB or the package. This technology is called Flip Chip. Package material, solder ball pitch and composition, and the internal interconnection method of the integrated circuit to the package vary greatly, but all of these components can generally be called area array packages (AAP).
During assembly of the printed circuit assembly (PCA), the solder balls on the AAP are soldered to corresponding circuit pads on the surface of the PCB. Standard practice is to design the PCB so the pads on the PCB and the MP are aligned vertically with each other.
The number of solder joints on a typical PCA may range from hundreds to tens of thousands per assembly. Assurance that the solder joints have been made correctly is critical to the performance and reliability of the PCA. With peripherally leaded components, inspection of the solder joints is either done by manual or automated x-ray inspection (MXI or AXI). X-ray is particularly suited to solder joint inspection since the heavy elements in solder (e.g., tin, lead, bismuth, silver, indium, etc.) appear in high contrast to the rest of the PCA, which is typically made of lighter elements (copper, carbon, hydrogen, sodium, oxygen, etc.) This high contrast in the x-ray image allows determination of the structural quality of each solder joint, and therefore, its connectivity and probable level of reliability.
In most x-ray inspection systems, the x-rays penetrate the PCA perpendicularly or nearly perpendicularly from one side of the PCA. An image of the solder joints is formed on an x-ray detector on the other side of the PCA. This image of the solder joints is then analyzed either visually or automatically by a computer. The solder balls (or columns) used in area array packages typically have a larger diameter than the connection pad on the PCA. Most conventional visual inspection methods are quite limited on area array packages, since the solder joints are between the IC package and the PCA and are hidden from view. X-ray inspection is a practical way of Inspecting the solder joints of area array packages. Many defects are easily detected with x-ray inspection. For example, missing solder balls (or columns), solder bridges (or shorts) between balls (or columns), voids in the solder ball (or column), and misalignment of the package are all capable of being detected using x-ray inspection.
Due to the vertical nature of the area array solder joint, the area where the solder ball (or column) connects to the PCA is directly underneath the solder ball (or column). When making an x-ray image of this type of solder joint, it is relatively difficult to evaluate the quality of this critical connection between the solder ball (or column) and the PCB pad, since the shadow of the material in the solder ball (or column) blocks the x-rays. This shading limits effective x-ray inspection of area array solder joints for opens since the elements of the solder joint (PCA pad, solder ball (or column), and package pad) are typically coaxial and arranged vertically to the plane of the PCA.
FIG. 1 illustrates a typical ball grid array 10 with a substrate 20 and solder balls 30 soldered to a printed circuit board 40 during x-ray inspection, wherein x-rays 50 are transmitted through the ball grid array package 10 and printed circuit board 40 to an x-ray detector 60. FIG. 2 illustrates an example of a typical ball grid array package 10 with an open solder joint 35 at the interface with the printed circuit board 40. FIG. 3 is an exemplary illustration of a typical final x-ray image of a ball grid array package as in FIG. 2. As will be readily apparent, the open solder joint 35 can sometimes be indistinguishable from other acceptable solder joints in the ball grid array package and printed circuit board assembly, due to the shading caused by the solder in the solder ball.
Tilting the printed circuit board assembly in the x-ray inspection system to mitigate the effect of this shading is usually ineffective, since adjacent solder balls (or columns) cause similar shading problems. Because of this shading, open solder joints are difficult to detect with x-ray inspection.
On many open solder joints on area array packages, solder that would typically be part of the connection with the printed circuit board pulls away from the printed circuit board and merges with the solder in the ball on the package. This results in a ball with a slightly larger diameter than that of acceptable solder joints. One method of automatic x-ray inspection that measures these diameters and compares them statistically to detect open solder joints is described In U.S. patent application Ser. No. 10/024,101, entitled “System and Method for Identifying Solder Joint Defects” by Tracy K. Ragland, which is hereby incorporated by reference.
This automatic inspection and statistical comparison technique is effective for detecting many opens, but has limited effectiveness on area array packages with solder balls (or columns) that are designed not to melt during the soldering process. In addition, the visual appearance of open and acceptable solder joints on the x-ray image being analyzed is almost indistinguishable, making confirmation of defects by a repair operator rather difficult. This technique is also susceptible to variations of solder ball diameters not caused by opens, such as voids and variation in the amount of solder used in the joints on the printed circuit board. These variations can cause incorrect indictment of acceptable solder joints or false acceptance of bad solder joints, reducing the credibility of this method in production manufacturing environments.
Some manufacturers have used oval pads on the printed circuit boards to enhance detection of opens, since part of the oval pad extends beyond the shadow of the solder ball (or column), which permits the x-ray inspection system to generate images of part of the printed circuit board pad. Like conventional round pads, the center of these oval pads is coaxial with the solder ball (or column) and package pad. Open solder joints tend to have a different appearance in the x-ray image than acceptable solder joints when oval pads are used.
FIG. 4 shows an exemplary x-ray image of an acceptable solder joint 70 with an oval pad. The acceptable solder joint appears uniformly dark due to thick solder shading the oval pad and the diameter of the solder ball equals X. FIG. 5 illustrates an exemplary x-ray image of an open solder joint 80 on an oval pad. The typical open solder joint on the oval pad appears lighter on the outer edges of the pad due to minimal solder remaining on the printed circuit board and the diameter of the solder ball is typically larger than X. As will be evident from FIGS. 4 and 5, the oval shape pads cause solder balls to form oval shaped solder joints 70 when the pad is properly bonded by the solder. And, conversely, the oval shaped pads permit detection of open solder joints as the associated solder image 80 remains somewhat circular.
However, as seen in FIG. 6, oval pads 84 cover more area on the surface of the printed circuit board than conventional round pads 82, resulting in difficulties in routing traces to the pads. FIG. 7 illustrates a small section of 36 pads of a typical round grid pad layout 86 on a printed circuit board or area array package. It is noted that the spacing of the pads 88 is regular for both rows and columns. FIG. 8 is an exemplary illustration of a typical trace routing layout with round pads 88 on a typical round grid pad layout 86 on a printed circuit board or area array package. It is observed that similar trace routing would be limited using oval pads 84.
Despite the improvement in detecting solder joints in area array packages utilizing oval pads, it would be desirable to have an improved system and method for improving the accuracy of solder joint inspection systems to accurately identify defective solder joints used to physically and electrically connect various printed circuit devices on printed circuit board while leaving more space available on the printed circuit board or area array package for trace routing layout.